Considerable evidence indicates that the acute and chronic actions of psychomotor stimulants (e.g. cocaine and amphetamine), as well as of other drugs of abuse, involve modulation of neurotransmission in mesolimbic and nigrostriatal dopamine systems. Our previous studies have revealed that a family of substrates for cAMP-dependent protein kinase, including DARPP-32, RCS (Regulator of Calmodulin Signaling, previously termed ARPP-21), and ARPP-16, are highly enriched in the basal ganglia, including the neostriatum and nucleus accumbens. Previous studies have indicated that a critical target for DARPP-32 is the serine/threonine protein phosphatase, PPL In recent studies we have found that RCS interacts with the Ca2+-binding protein calmodulin and in turn regulates the serine/threonine protein phosphatase, calcineurin (or PP2B). In other studies, we have found that ARPP-16 is likely to regulate the stability of GAP-43 mRNA. In Project III, we propose to study the role(s) of RCS and ARPP-16 in mediating or modulating the actions of psychostimulants. In addition, we propose to study the roles of the three isoforms of PP1 (PPla, (3 and y) in the actions of psychostimulants. The Specific Aims are: Aim I: Characterization of RCS - We will analyze in RCS knockout mice, several aspects of animal behavior and physiology that are modulated by psychostimulants. These studies will include acute and chronic motor-stimulant properties and drug reinforcing properties of cocaine and amphetamine in RCS mutant mice. We will also characterize molecular targets for RCS that may be involved in mediating the actions of psychostimulants. Studies will include analysis of RCS and regulation of calcineurin, and of RCS and regulation of other calmodulin-binding proteins. Aim II: Characterization of ARPP-16 - We will analyze in ARPP-16 knockout mice, several aspects of animal behavior and physiology that are modulated by psychostimulants. These studies will include acute and chronic motorstimulant properties and drug reinforcing properties of cocaine and amphetamine in RCS mutant mice. We will also characterize the regulation of ARPP-16 by phosphorylation at novel sites and study the phosphorylation of these sites in response to psychostimulant treatment. Aim HI: Characterization of PP1 isoforms - We will investigate the role of PPla, (3 and y isoforms in the actions of psychostimulants. These will include acute and chronic motor-stimulant properties and drug reinforcing properties of cocaine and amphetamine in RCS mutant mice. Results from our studies will complement other Projects of this Program Project grant. Together, these studies will hopefully lead to elucidation of the biochemical pathways through which drugs of abuse act in the brain, and to an increased likelihood that therapeutic agents will be developed that will prevent or reverse molecular adaptations within these pathways.